Help me Mastodon! #Apple #Mac question. I have a MacMini running OS 10.5.15 Catalina. I need to keep using this outdated OS because I need to use an old version of QuarkXPress for work.

I’m trying to install OS Monterey on an external SSD but I can't boot from it because I can't for the life of me find Startup Security Utility in Catalina to allow that to happen. Does anyone know where the hell it is? It *should* be in Utilities but it's not.